In the recent network environment, there is an ever increasing trend of a crime aiming at security vulnerability such as data leakage and service interruption by hacking.
For example, a radio base station apparatus of a mobile communication system such as LTE (Long Term Evolution) supports an important infrastructure of a society and is expected to have a high reliability. The security vulnerability in the radio base station apparatus has a great socioeconomic influence and is a problem to be worried about significantly. Accordingly, a very strong protocol is desired in terms of wiretapping prevention and falsification detection, and the IP Security Protocol (hereinafter, described as IPsec) is widely used as a protocol realizing security in the IP (Internet Protocol) layer.
One security function provided by IPsec is data encryption using a technique called a block cipher. For example, IPsec prepares a CBC (Cipher block Chaining) mode. The CBC mode attaches a code called IV (Initial Vector) to a header of each encrypted packet and performs encryption after exclusive OR of IV and a plain text of a head encrypted block, and thereby realizes communication having an improved encryption strength.
RFC4301, “Security Architecture for the Internet Protocol”, December 2005
RFC4303, “IP Encapsulating Security Payload (ESP)”, December 2005
RFC4306, “Internet Key Exchange (IKEv2) Protocol”, December 2005
Meanwhile, there are a free-call service application using a short packet (e.g., approximately 150 bytes) and an online game service, as services provided through the mobile communication system.
For the short packet too, IV is attached to a header for each encrypted packet. For example, in the case of AES (Advanced Encryption Standard) which is one of encryption algorithms, IV having the same size as the length of the encryption algorithm (16 bytes) is attached. A fixed overhead due to the IV attachment increases communication traffic to become a burden on a communication line. For example, the increase of the communication traffic between the radio base station apparatus and a security gateway in the mobile communication system (transmission system) may cause a network bottleneck. Note that, it is well known that in the transmission system, the security of encrypted data becomes vulnerable in the case of not using IV as in ECB (Electronic Codebook) mode prepared by IPsec or the like, compared with the case of using IV as in the CBC mode.